Tankless Boiler with Radiators

I am in need of replacing my oil fired boiler, it heats my hot water too. I live in Delaware in a 2200 square foot, 1900's farmhouse with original radiators. I am on well water, and I think it to be rather acidic. I am planning to replace the oil fired with a natural gas boiler.

I would like to know if a natural gas, tankless boiler, will work WELL with a radiator system. I am told by some heating people yes, and some no. The no people I think, if I understand them correctly, say the efficiency, of the system, will be diminished greatly, because radiators need a good amount of water to be to be heated and at a very high temperature, before it circulates through the radiators, and a tankless boiler will not do this properly for radiators to operate properly.

A heating person made the comment that the system he would propose has a low loss header, built in to it, or maybe it is an accessory you purchase separately. He said this makes everything ok with my old radiator system, and I need not be concerned.

Thanks, in advance, for any input, and until someone can straighten me out, I shall remain,

Many tankless systems' warranty is voided if you use them for space heating I've been told. They are designed for sporatic use, not constant use. You'd need a heat exchanger if you were going to use this as your potable water source as well, since generally, it's not a great idea to have long lines running to radiators that you then bath with. The heating system needs to be protected from the fresh supply of oxygen you'd get in an open system verses a closed one as well.

As noted in the boiler section of the forum, your existing boiler is probably at least 2x oversized. Personally, I'd go with a boiler and indirect. Trying to make a device do something it wasn't designed for often ends up being more expensive in the long run, and may not work correctly.

I may have used "tankless" in the title, incorrectly. I will edit it to read "condensing wall mounted gas boiler with radiators."

The system I am speaking of is a 175,00 BTU condensing wall mounted gas boiler by Rinnai model Q175S. The Minimum/Maximum Gas Rate Input BTUs (Heating): 35,000 - 175,000 (NG) It is what they call a combi unit which is designed for heat, and domestic hot water. I am just not sure how compatible this is with the old radiators.

I am not sure, but I think the BTUs, on my oil system, are at 144,300, because some of that accounts for the boiler heating the hot water? So far, I have only gotten two heating people who have asked for the dimensions of the existing radiators, and the number of passes in them, everyone else, I think is just going by what it says on the id plate, which, as you indicated, could actually be wrong. Not sure how you tweak the Rinnai but it looks like one model can accommodation any where from 35,000 to 175,000 BTUs.

What would be the difference between an indirect, and an independent gas hot water heater? Is one better than the other, would one cost considerably more, than the other?

That is WAY too big and you'd never get the efficiency out of it. You're better off with a smaller unit, but you won't know until you run the numbers. The last thing you want to do is put in an even bigger unit. You do not need extra capacity to heat the water unless you run something like a spa where they're using water all the time. On a typical setup with an indirect, the tank supplies most of the hot water, then the boiler turns on to reheat it. It is set up as a priority zone, so when it needs heat, it stops space heating. There are lots of wall-hung boilers, and it's likely that something in the 60-90KBTU would be more than enough.

Run the numbers...don't let someone just copy what you have or you'll be sorry. A too large boiler never reaches its effiency peak, and ends up shortcycling, too.

If the 5-plate oil burner wasn't being short-cycled to death there is probably sufficient thermal mass to the radiator system to be able to run a boiler that has a min firing rate of 35K efficiently, but I agree that a much smaller boiler is probably called for- your 99th percentile design condition heat load can easily be lower than 35 KBTU/hr if you have insulated walls and storm-windows (or double-panes). MOST 2200' homes in the lower-48 of the US could be heated comfortably with a smallest-in class 50-60K (max input) modulating condensing boiler. The Rinnai combi is a glorified tankless hot water heater, but peak hot water BTU/hr rates are typically several times that of a space heating load on a house that size, particularly at the modest outside design temperatures of the mid-Atlantic states.

To get a handle on what your true space heating loads are based on the performance of your 5-section oil-beastie with the tankless DHW coil, based on your fuel use or "K-factor" (often stamped on oil bills) download the FSA calculator from the NORA website and play with it a bit. It's not a perfect tool, and can crash fairly easily, but the results aren't crazy, based on boiler models developed at the Brookhaven Nat'l Lab. All good heating system designs start with a room-by-room heat load calculation, but if the room-to-room temperatures have been reasonably balanced using the old boiler, the whole-house heat load based on fuel use (from the FSA calculator or other) is enough information to size the boiler.

If your design condition heat load is under 40K and you have sufficient radiation to be able to heat the place at the even the 95th percentile outside temperature with 140F water you might consider using a gas-fired tank type combi heater (condensing or otherwise.) If you need higher peak temps for the design-condition load you're probably better off with a 50K mod-con and an indirect fired water heater running as a "priority zone", and tweaking the outdoor-reset curves to run low-temp to the radiators most of the time.

An indirect will be more efficient than non-condensing standalone tank heater since it doesn't have a center-flue sucking heat out of the tank during standby. It also improves the AFUE of non-condensing boilers by giving it a higher duty-cycle (=less time spent idling in standby.)

The notion that "...the efficiency, of the system, will be diminished greatly, because radiators need a good amount of water to be to be heated and at a very high temperature, before it circulates through the radiators" is bunk. Since the thermal mass of those radiators is all inside of conditioned space and the Rinnai or any mod-con is both tolerant of cool water returns (unlike an oil boiler) there's no need to heat the boiler to high temp before circulating the water into the radiators, and the average temp of the water will stay lower (== more condensing efficiency). And the thermal mass of that water guarantees that you get fewer but longer burns out of the boiler, lowering it's losses to flue-purges & ignition cycles.